11.1 How do I change the included text prefix in mail/news followups?

If you read mail with Rmail, set the variable mail-yank-prefix. For Gnus, set message-yank-prefix. For VM, set vm-included-text-prefix. For mh-e, set mh-ins-buf-prefix.

For fancier control of citations, use Supercite (see the Supercite Manual in The Supercite Manual).

To prevent Emacs from including various headers of the replied-to message, set the value of mail-yank-ignored-headers to an appropriate regexp.